Liquid-jet guided laser technology, sometimes referred to as Liquid MicroJet (LMJ), couples the laser focus into a small liquid-jet, for example, through a focusing lens. This coupling takes place in a coupling unit. The coupling unit can include a metal chamber on the side of the focusing lens that is closed with a laser protection window. The opposite side the chamber carries a nozzle. Liquid provided to the coupling unit flows between window and nozzle and leaves the nozzle in form of a liquid-jet. The energy of the laser spot in the focal plane is captured inside the liquid-jet and guided to the work piece through internal reflection. This method eliminates the necessity to control the distance of the work piece precisely because the required energy to perform the processing is available throughout the laminar length of the liquid-jet. Any liquid that provides suitable light guide capabilities can be used to form the liquid-jet.
The laminar length of the liquid-jet can be increased to extend the working distance of the process by providing an assist gas to the liquid-jet. The assist gas is guided as a direct boundary layer to the liquid-jet in order to reduce the resistance between liquid and ambient air and thereby increase the laminar length of the liquid-jet. Thus the liquid jet is surrounded by the assist gas and inside the coupling unit, the assist gas is directed toward to the liquid jet. For example, the assist gas can enter the coupling unit in the horizontal plane toward the liquid-jet that is travelling in the vertical plane. The assist gas and the liquid jet then leave the system, with the liquid jet in the middle surrounded by the assist gas.
There is dependency between the assist gas and the liquid jet. For example, the pressure and flow properties of the assist gas can be selected to optimize the laminar flow of the liquid jet. Other operating conditions of the assist gas can adversely affect the liquid jet. For example, a high pressure of the assist gas can shorten the laminar flow of the liquid jet, and an even higher pressure of the assist gas can destroy the liquid jet. The assist gas can also interrupt internal reflection of the laser beam in the liquid jet thereby affecting laser-processing power.